Phosphoglycerate kinase exists in altered form in senescent rat liver and muscle. It is proposed to determine if the altered form also is present in other old tissues such as brain, kidney and heart. In addition, the type of phosphoglycerate kinase these tissues contain (liver vs. muscle) will be determined by immunotitration with antisera prepared to both the liver and muscle enzyme. The turnover time of the enzyme in these tissues in young and old rats will be determined. If altered enzymes are found in tissues where turnover is slowed with age, the results will support the thesis that slowed turnover results in conformationally altered enzymes. To show that "old" phosphoglycerate kinase is a conformational isomer of "young" phosphoglycerate kinase, both forms of the muscle enzyme will be subjected to unfolding-refolding experiments. If they refold to the same product, then they were originally conformational isomers. Since protein synthesis is slowed dramatically in old Turbatrix aceti, a free-living nematode, a cell-free protein synthesizing system will be prepared from young and old organisms. The latter preparations will be examined for deficiencies which may be responsible for reduced synthesis.